To the lyricists, vocalists, and emcees -- how do you balance the writing / recording processes?

All vocalists have different quirks, strengths, weaknesses, and writing styles, obviously. But where some can listen to a beat and jump in the booth that day, I need to take my time to write every single bar, to the breath. When it comes to writing, speed just isn't my strong suit, so when I read about those of you who write your songs on the spot -- I get maaaad jealous!!! I want to know how you all do it!

XW